Vadose Zone Modeling In RCRA Closure - Ohio

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Vadose Zone Modeling in RCRA Closure " This Policy Does Not Have the Force of Law" January 7, 2005 Division of Hazardous Waste Management Ohio Environmental Protection Agency

Table of Contents 1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1.1 Applicable Uses of Mod els for RCR A Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.0 Tier I: Leaching Factors Screening Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Orga nic Cons tituents: Leach ing Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Applicability/Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Tier I: Evaluation of Organic Constituents Leaching to Ground Water . . . . . . . . . . . . . . . . . 2.3.1 The Tier I Process for Organic Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Step 1. Determine a Chemical-Specific LF gw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Step 2: Determine the Vadose Zone Material and its Thickness. . . . . . . . . . . . . . . 2.3.4 Step 3: Ensure that the Measured Soil Chemical Concentrations Do Not Indicate the Presence of NAPL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 Step 4: Com pare Chemical-Specific Leaching Factor with Critical Leaching Factor 2.3.6 Tier I Process Sum m ary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Tier I: Evaluation of Inorganic Constituents Leaching To Ground W ater . . . . . . . . . . . . . . . 2.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Application/Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Be havior of Specific Meta ls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5 Tier I Optio ns for Evaluatin g Leaching of M eta ls . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 20 22 25 25 26 27 27 28 30 31 31 31 32 34 37 3.0 Tier II: Modeling Ground W ater Pathway Viability Using Generic Default Parameters and a Dilution and Attenuation Fa ctor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.1 Specific Model Assumptions for Generic Unsaturated Zone Modeling . . . . . . . . . . . . . . . . 41 3.1.1 Determ ining the Partition Coefficient (K d) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.1.2 Determ ining the appropriate Partition Coefficient (K d) . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.2 Criteria for A sse ssing G ene ric M ode l Res ults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 4.0 Tier III: Modeling Us ing Both G ene ric Defaults an d Site-Specific 4.1 Site-specific Input Param eters . . . . . . . . . . . . . . . . . . . . . 4.2 Determ ining Appropriate Dilution Fac tors . . . . . . . . . . . . . 4.3 Criteria for A sse ssing G ene ric M ode l Res ults . . . . . . . . . . . . . 48 48 50 53 5.0 Mo deling Requirem ents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Conceptual Model Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Site Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Model Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Model C riteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Code-Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Consistent Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Model Specific Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Model Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Model Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Se nsitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Model Report Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 55 55 56 57 57 57 58 58 59 59 59 D ata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Appendix A - Soil Hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.1 Soil Hydrology Background Information . . . . . . . . . . . . . A.1.1 Contam inant M ovem ent through the Soil . . . . . A.1.2 The Soil System . . . . . . . . . . . . . . . . . . . . . . . . . A.1.3 Soil Texture . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.1.4 Soil-W ater Relationships . . . . . . . . . . . . . . . . . . A.1.5 Soil W ater Movem ent . . . . . . . . . . . . . . . . . . . . A.1.5 .1 Saturated Flow . . . . . . . . . . . . . . . . . . . A.1.5 .2 Flow of W ater in Unsaturated Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 70 71 72 74 77 81 81 83 Appendix B - Appropriate Use of Com mon D efault Parameters for Vadose Zone Models . . . . . . . . . . . 85 B.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

B.2 B.3 B.4 B.5 Dry Soil Bulk De nsity (D b) Pa rticle D ensity (D s) . . . . Porosity . . . . . . . . . . . . . W ater Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 86 86 87 1: Tier I Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2: T ier I Proces s for inorganic con stituen ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3: Tier II Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4: Tier III Process Conceptual Site Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5: Conceptual Site Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.1: The three components of the unsaturated zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2: Soil texture classification diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.3: A typical pore water retention curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.4: Typical water retention curves for sand, silt, and clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 16 17 18 21 73 74 79 80 List of Figures Figure Figu re Figure Figure Figure Figure Figure Figure Figure List of Tables Tab le Table Table Table Table Table Table Ta ble Table Table Table 1: Param eter Defaults Us ed to Deve lop the Gene ric Leaching Fac tors . . . . . . . . . . . . . . . . . . . . 2: Critical Leaching Factors for Ground W ater Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . 3: Generic Kd Values fo r Selected Inorg anic Constitue nts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4: Methods to Determine Comm on Input Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.1: Factors Affecting Chem ical Movem ent in the Vadose Zone . . . . . . . . . . . . . . . . . . . . . . . . . A.2: Table of Pore-Size Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.3: P hysical Properties of S oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.4: Co nversion Fac tors for C om m on P ressure an d Head Units . . . . . . . . . . . . . . . . . . . . . . . . . A.5: T ypical values for the irreducible wate r conten t and air en try press ure fo r com m on soils . . . A.6: Typical saturated hydraulic conductivity values for various soil and rock types . . . . . . . . . . . A.7: Representative shape factors (n) and corresponding m values for various soil types . . . . . . 26 29 45 53 70 75 76 78 79 82 84 List of Equations Equation Eq uation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation Equation 1: Ge neric Leaching F actors (Che m ical-Specific) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2: Soil Satu ration Lim it . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3: Partitioning Con stant (K d) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4: Organic Carbon Partition Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5: Summ ers Model (Ground W ater Dilution) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6: Simplified Summ ers Model (Clean Background) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7: Dilution Attenuation Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8: Mixing Zone Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.1: P oros ity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2: Darcy's Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.3: Hydrostatic Pressu re . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.4: E ffec tive Po rosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.5: S atura ted C ond uctive and Intrinsic Perm eab ility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.6: Intrinsic Perm eab ility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.7: Richard's Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.8: van Genuchten Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.1: Dry So il Bulk D ens ity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.2: Poro sity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.3: Perc ent Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.4: Volumetric W ater Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.5: W ater-filled Pore Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.6: Percentage of W ater in Pore Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 28 32 43 49 50 50 52 75 81 81 81 83 83 83 83 86 87 87 87 88 88

Acronyms ACL ASTM COC CPRG DHWM MCL NAPL OAC ODNR RCRA REDOX SPLP TCLP UCL Alternate Concentration Limit American Society for Testing and Materials Constituent of Concern Closure Plan Review Guidance (an Ohio EPA document) Division of Hazardous Waste Management Maximum Contaminant Levels Non-Aqueous Phase Liquid Ohio Administrative Code Ohio Department of Natural Resources Resource Conservation Recovery Act Reduction Oxidation Potential Synthetic Precipitation Leaching Procedure Toxicity Characteristic Leaching Procedure Upper Confidence Limit Variables Parameter Definition A Ca Cgw Cp Csat Csol Csolid CLFgw DAF d d" exp foc g h h2-h1 H’ i K Kd Kh Koc Kow KP Cross-sectional area perpendicular to flow Upgradient concentration of the pollutant in the aquifer (if any) (µg/ml) Concentration of the contaminant in the saturated zone (µg/ml) Contaminant concentration in the soil pore water (µg/ml) Soil saturation limit (mg/kg) Concentration of a metal dissolved in the soil solution Concentration of a metal absorbed to the solid material Critical Leaching Factor (kg/L) Dilution Attenuation Factor Depth of mixing zone (m) Aquifer Thickness (m) Inverse of the natural log Fraction of organic carbon Acceleration of gravity or grams Pore water pressure head (cm) Change in head per unit length Henry’s Law constant for the COCs (dimensionless) Hydraulic gradient (m/m) Horizontal hydraulic conductivity (m/yr) Partitioning constant for the soil Koc X fOC (cm3/g) Unsaturated hydraulic conductivity (cm/day) Organic carbon coefficient (Kg/L) Octanol-water partitioning coefficient Partitioning coefficient

Ks Kv L LFgw ne q Qr Qgw r S S T v w )P 6 : N Db Df Ds 2a 2m or 2v 2r 2s 2w -RA Rm M2/Mt Mh)/MZ Saturated hydraulic conductivity Vertical saturated hydraulic conductivity Source length parallel to ground water flow (m) Leaching factor (kg/L) Effective porosity Flux Volumetric flow rate of infiltration (soil water) to the aquifer (cm3/d) Volumetric flow rate of ground water beneath the contaminated area (cm3/d) Infiltration rate (meters/year) Irreducible water content Solubility in water (mg/L) Temperature Linear pore water velocity Length of source perpendicular to ground water flow Change in hydrostatic pressure Intrinsic permeability (cm2) Dynamic viscosity Porosity Dry bulk density (gm/cm3) Fluid density Particle density Fraction of air filled porosity Water-filled porosity or water content Irreducible water content Volumetric water content Fraction of water filled porosity Air entry pressure Matric potential, suction head, or pore water pressure Change in volumetric water content through time (cm 3/cm3) Change in head with depth (cm) Glossary Absorption - Partitioning of a dissolved species into a solid phase. Adsorption - Partitioning of a dissolved contaminant onto a solid surface. Advection - Fluid migration induced by hydraulic gradients. Albedo - Fraction of sun light reflected by the atmosphere (never reaches Earth's surface). Anisotropy - The condition wherein soil characteristics vary differently in different directions due to an arrangement or alignment pattern within the soil.

Bulk Density (Db) - The oven-dried mass of a fixed volume of soil divided by the original sample volume. Calibration (of a model) - A process that involves varying input parameters to result in a critical output parameter (e.g., ground water recharge rate) that is more reflective of natural conditions. Capillary Fringe - A zone above the water table into which moisture is pulled due to capillary action. Cation Exchange Capacity - The excess of cations in solution adjacent to a charged surface that replaces other cations already absorbed to that surface. The sum total of exchangeable cations that a soil can absorb. Complexation - Any combination of cations with organic molecules or cations with anions containing free pairs of electrons Conceptual Site Model - A generalized picture that sums up the contaminant sources, contaminant movement mechanisms, routes, end points (receptors), and other factors possibly affecting contaminants at a site. Hydraulic Conductivity (K) - hydraulic conductivity is not a tangible measurable characteristic but rather is defined by the equation: K -Q / (A dh/dl) where Q is the discharge, A is the cross sectional area, and dh/dl is the hydraulic gradient. The units of K are L/T. Constituents of Concern - Organic or inorganic contaminants that may have contaminated environmental media resulting from the activities at a facility that generated hazardous waste and/or hazardous constituents. Co-Solvent Effects - A situation where the presence of another contaminant (e.g., an organic solvent) makes a contaminant more mobile than would be expected if it were the only contaminant present. Degradation - Biological or abiotic (e.g., redox or hydrolysis reactions) breakdown or transformation of organic chemicals. Diffusion - A transport process in which chemicals migrate in fluid due to concentration gradients, moving from areas of higher concentration to areas of lower concentration. Dilution Attenuation Factor - Accounts for dilution of soil pore water when it mixes with ground water. Various equations are used to estimate this factor.

Disconnectedness - A catchall term used in the SESOIL model that includes effects of hysteresis and other factors that affect contaminant movement (based on soil type). Dispersivity - A characteristic of the geological medium attributed to tortuosity and heterogeneity that affects mechanical mixing of chemicals during advection. Effective Porosity - This is the interconnected portion of the total porosity, the part that is actually available for flow (i.e., contaminant migration). Flux - The amount of water per unit volume that passes through a defined area. Fraction of Organic Carbon - This is the carbon in the soil that is made up of decaying plant and animal matter, humus, etc. It is differentiated from inorganic carbon (typically in calcium or magnesium carbonates) which does not have the same impact on contaminant movement. Generally the dominant retarding mechanism for contaminant movement in the vadose zone. Hydraulic Conductivity - a measure of how easily water flows through (or can be pumped from) an aquifer. Hydraulic Gradient (dh/dl) - The change in water table height (dh) over some distance (dl). The hydraulic gradient describes the direction and magnitude of ground water flow. It does not describe the velocity of ground water movement. Hydrologic Cycle - The exchange of water among the ocean, atmosphere, and land by such processes as evaporation, precipitation, surface runoff, and groundwater infiltration. Hydrolysis - The addition of water to a molecule. Infiltration - The amount of water that enters the vadose zone (i.e., from rainfall or irrigation). Infinite (vs. Finite) Source - Used as an input assumption in some vadose models. In a finite source, the concentration of soil contamination depletes over time as contamination migrates and degrades, etc., whereas an infinite source, the concentration remains relatively constant over time such as is found in a former landfill, surface impoundment, etc. Isotropy - The condition wherein soil characteristics vary in a similar way regardless of direction. Kd - Distribution coefficient, the ratio of contaminant concentration associated with the solid to the contaminant concentration in the surrounding aqueous solution when the system is at equilibrium.

Koc - Partitioning coefficient between organic liquid and organic carbon. Kow - Octanol water partitioning coefficient. Measures a contaminant's affinity for polar or non-polar solutions. Leaching - The removal of constituents from a waste by the action of percolating fluids. Ligand - Anion (e.g., SO4 2-) or molecule with which a cation (e.g., Pb 2) forms complexes. Mass Balance - Evaluation of contamination using the total raw amount of the contaminant(s), not just the concentration in media. Matric Potential - Also called the suction head or pore water pressure, the force of adhesion that draws water into pore space. The matric potential is inversely proportional to the pore-size of a soil. Miscible Displacement - A contaminant transport mechanism whereby retardation of chemical movement is primarily driven by exchange reactions between inorganic contaminants and soil particles that is affected by chemistry of the solute, the pore solution, and the soil. Mixing Zone - The vertical depth in a aquifer in which a contaminant is expected to mix (and thereby dilute). Non-Aqueous Phase Liquid - Liquid "free product" present or potentially present in the vadose zone. Generally evaluated by comparing soil contaminant levels to soil saturation limits. Partition Coefficient - The term that linearly relates the concentration of a contaminant in the water phase to that in the solid phase by the following equation Csolid Kd Cwater. Partitioning - A transport process in which chemicals are distributed between solid, liquid, and gas phases, depending upon solubility, sorption, and vapor pressure characteristics. Permeability - Ease with which a soil can transmit fluid when saturated with that fluid (i.e., water). Pore Water Velocity - The mean water flow rate in the soil pores, sometimes refered to as the Darcian velocity. Porosity - The ratio of void space to the unit bulk volume of soil. It may be filled with water and/or air. Recharge - The amount of infiltration water that makes it through the vadose zone

down to the ground water table. Retardation - Factors limiting pore water velocity. Runoff - Water (i.e., from rainfall or irrigation) that does not infiltrate into the vadose zone but instead "runs off" into surface water. Sensitivity Analysis - The process of varying model input parameters over a reasonable range (range of uncertainty in the value of the model parameter) and observing the relative change in model response. Soil Saturation Limits - A calculated value that estimates the level at which a contaminant will partition from the aqueous phase into a Non-Aqueous Phase Liquid. Solubility - A measure of a theoretical maximum level a contaminant will dissolve in an aqueous phase before the contaminant is available as "free product." Sorption - The chemical or physical process of sorbing chemicals to solid surfaces. Source (instantaneous vs. continuous) - An input term for some vadose models that differentiates between a spill and something more like a long term leak or impact from a relatively infinite source like a landfill. Species - The form (or valence state) of a dissolved ion, element, or molecule as it is present in a particular solution. Tortuosity - A measure of the non-linear pathway that water molecules taken within aquifer or vadose zone materials. Vadose Zone - The section of the Earth where soil water, soil particles, and soil gas exist in equilibrium. This zone lies between the ground surface and the top of the water table. Vapor Pressure - The pressure exerted by a vapor in equilibrium with its solid or liquid phase. Volumetric Water Content - Volume of water per unit volume of bulk soil.

Preface This document was written primarily as a guidance for developing and reviewing vadose zone models that are generated by facilities to support that wastes left in soil, after meeting direct contact risk standards, will not pose a significant threat to ground water resources. In this guidance, this analysis is divided into three tiers. The tiers are characterized by increasing levels of site-specific information that is necessary to make a leaching demonstration. Division of Hazardous Waste Management personnel will use the guidance primarily to evaluate input parameters and assumptions used to develop vadose zone models for RCRA closure sites in Ohio. Division personnel may also choose to use a vadose zone model to double-check the results submitted by a facility (using the same or a different model).

Section 1.0 Introduction

1.0 Introduction The vadose zone is defined as that section of the earth where soil water, soil particles and soil gas exist in equilibrium. This zone is important because it is there that contaminants are often introduced into the environment and eventually are transported to groundwater. An understanding of how chemicals can migrate through the vadose zone is necessary for environmental professionals to predict the impact that contamination may have on human health and the environment. Fate and transport modeling of contaminants in the subsurface has been successfully applied to numerous environmental sites throughout the country. Models have been used to predict the time of travel and concentration of contaminants in groundwater to some point, such as a unit boundary well. However, fate and transport modeling of chemicals in groundwater has had only limited acceptance at Resource Conservation Recovery Act (RCRA) closure sites, mainly because of the uncertainties and validation practices associated with numerical modeling. In fact, guidance from the U.S. EPA has specified that only limited modeling be performed at a RCRA closure site and only under strict conditions (See Elizabeth Cotsworth Memorandum, reproduced in Ohio EPA, Closure Plan Review Guidance (CPRG), 1999 and later revisions). Ohio EPA, Division of Hazardous Waste Management (DHWM) has allowed the use of vadose models to determine the leaching potential of contaminants that remain in soils at a closure site. In addition, groundwater modeling for engineering purposes and for the determination of an Alternate Concentration Limit (ACL) has also been accepted. Engineering applications include a determination of the number and location of pumping and extraction wells, and to determine extraction rates for ground water pump and treat systems. Soil vapor extraction systems, air sparging systems, and biological treatment systems have also been successfully modeled. The benefit of modeling is that pre-optimization of remediation systems can be performed in the office and not in the field, saving time and lowering costs. Numerical models have also been developed that can predict the rate of intrusion of vapors and the concentration of these chemicals that can enter into buildings. Vapor intrusion into buildings is recognized as a potential pathway of human exposure and these models can also be used to judge whether vapor intrusion is a viable pathway. All of these examples of model use are important; however, the scope of this guidance will be limited to the movement of chemicals through the unsaturated zone to the upper most aquifer to assure that leaching of chemicals is not a concern. Vadose zone leaching models are available that cover a wide range of applications ranging from screening-level analytical models to advanced numerical models. The Vadose Zone Modeling for RCRA Closure Page 12 of 88

approach taken in the guidance document focused on the methodology outline in ASTM and U.S. EPA guidance on chemical transport through the unsaturated zone. The application of any model will depend upon the RCRA closure objectives. Usually data collected from normal RCRA closure investigations is insufficient to support the site specific vadose zone modeling. One of the most important parts of the Data Quality Objective (DQO) process is the acquisition of a sufficient quantity of quality data as input parameters for a model. Therefore, sampling activities for a RCRA closure should reflect the DQOs needed to complete any anticipated model. There are several useful references for data collection to support subsurface investigations and modeling including, the U.S. EPA publication, Site Characterization for Subsurface Remediation (1991) and Handbook of Vadose Zone Characterization and Monitoring (Wilson et al, 1994). In addition, data quality documents that should be consulted are U.S. EPA's DQO publications that can be found at US EPA's website: http://www.epa.gov/quality/qa docs.html. This document is divided into six sections. Section 1 is an introduction, Section 2 outlines the Tier I process, Section 3 describes the Tier II process, Section 4 describes the Tier III process and Section 5 outlines the basic approach to using numerical models and discusses reporting requirements. Section 6 is a list of references and other potentially useful sources of information. Appendix A provides background information (i.e., American Society for Testing and Materials, ASTM, methods) on subsurface hydrology and other parameters of interest. Appendix B lists important physical properties for a wide variety of organic chemicals. Appendix C lists conventional methods to determine important soil physical parameters, such as dry bulk density. 1.1 Applicable uses of leaching models Unsaturated zone modeling has wide applicability in environmental sciences and is often successfully coupled with saturated zone modeling. Because of the limitations imposed by U.S. EPA on modeling at RCRA closure sites (See Elizabeth Cotsworth Memorandum, as re

K s Saturated hydraulic conductivity K v Vertical saturated hydraulic conductivity L Source length parallel to ground water flow (m) LF gw Leaching factor (kg/L) n e Effective porosity q Flux Q r Volumetric flow rate of infiltration (soil water) to the aquifer (cm 3/d) Q gw Volumetric flow rate of ground water beneath the contaminated area (cm3/d) r Infiltration rate (meters/year)

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